Bingo. People use Tik Tok because their friends, and their friends’ friends (and so on) use Tik Tok.
How they got there, I don’t know. I suspect it was probably something silly like “My parents don’t know about Tik Tok, so let’s use that.”
Facebook’s Tik Tok competitor is probably going to be about as great as Google+ ended up being, and for the same reason- everyone who already uses Tik Tok probably isn’t so big into Facebook, so adding Tik Tok type functionality to Facebook isn’t going to do much for 38 year old housewives and the like.
Now if the government bans Tik Tok, then someone else will either figure out why Tik Tok hit it big, or they’ll just get lucky and not know why. Either way, something will supplant it, and I doubt it’ll be Facebook’s attempt.
Although I will note, that Facebook’s Tik Tok competitor probably will not be “Facebook” branded, but “Instagram” branded. Recall Instagram is owned by Facebook and trends decidedly younger. Instagram stories tried to take on Snapchat (after Snapchat made an interface mistake, IIRC) and did indeed take a decent chunk, but Snapchat is still kicking.